This invention refers to the general field of the molding machines, such as injection molding, die casting and press molding machines; in particular the invention is directed to a clamping device for clamping the mold-support platens of a molding machine, suitable for allowing the tightening and the adjustment of the platens in a plurality of clamping positions for molds of different thickness and size.
The invention is also directed to a molding machine of the type previously referred to, provided with a similar adjustable clamping device.
In several molding fields, for example in the injection molding field for plastic materials, there exist several types of molding machines or presses which make use of different mechanisms or clamping systems for locking the mold support platens, which allow a certain degree of adjustment for molds of different thickness and size.
Usually such molding machines comprise a stationary or fixed mold-support platen, and a movable mold-support platen which, in the closed condition of the machine, tightly clamp the two mold members by means of forces exerted by hydraulic cylinders directly or indirectly acting on appropriate tie bar which extend between the platens of the machine.
Molding machines provided with mechanical clamping devices are known, for example, from U.S. Pat. Nos. 2,916,768, 5,192,557, 5,320,517, WO 98/51468, and U.S. Pat. No. 6,200,123.
In particular, U.S. Pat. No. 2,916,768 describes an injection molding press, in which the tie-rods for transmitting the clamping force extend from a movable platen supporting a first mold member, towards a fixed platen supporting a second mold member, and in which each tie rod comprises at one end, a toothed head designed to engage with a corresponding clamping sleeve on the stationary platen, in a position aligned with the tie bar.
The clamping sleeve is simply made to rotate by means of a worm drive, to engage and disengage with the toothed head of the tie bar in a single coupling position.
The dispacement of the movable platen towards and away from the fixed platen is performed by means of an appropriate hydraulic cylinder, while the rated clamping force for closing the mold is produced by separate hydraulic cylinders axially aligned to the tie bars.
In presses on molding machines of the aforesaid kind, in order to change the relative position of the two platens supporting the two mold members, and to adapt their disposition to molds of different thickness or size, it is necessary to change and adjust the axial extension of the tie bars by means of complicated operations which involve prolonged machine downtimes, resulting in a consequent low productivity.
The adjusting operations for adapting the press to molds of different thickness or size, as well as the time required for the opening and closing operations, have therefore considerable repercussions on the entere working cycle time.
WO 98/51468 describes an injection molding machine of the type previously referred to, in which use is also made of clamping sleeves, on the fixed platen of the machine, and in which each clamping sleeve is connected to the piston of a hydraulic cylinder designed exclusively for generating the clamping force necessary for closing the two half molds.
Each clamping sleeve is consequently provided with a single set of teeth to engage with a toothed head of a respective tie rod in a single axial position.
In this type of machine also, the adjustment of the clamping position is obtained by means of wholly conventional systems, requiring again an adjustment of the axial extension of the tie bars as in the previous case; once again, all this entails time consumption and complexity in handling the machine, with a consequent negative influence on the working cycle.
The previously proposed solutions therefore involve complicated operations for carrying out all the necessary adjustments of the tie bars, as well as are time consuming for closing and clamping the molds, which are unsuitable for fast cycle molding machines.
In molding machines there is also a need to make automatically adjustments, and to carry out the closing and clamping of a mold, as quickly as possible in order to reduce the downtimes in a production cycle.
In order to partially solve this problem, U.S. Pat. No. 5,320,517 proposes some solutions according to which use is made of clamping jaws fitted into the piston of a hydraulic clamping cylinder to engage a threaded portion of a tie bar, and in which the axial position of the tie bar is determined by an adjustable stop member screwed into a threaded bush.
During the clamping of the platens, after the tie bars have come into contact with the stop members, the clamping jaws are tightened against the threaded ends of the tie bars, while the piston members slowly move within their respective cylinders.
Upon completion of the clamping step, the hydraulic cylinders are fed with pressurised oil to generate the required clamping force.
Even though this solution permits an axial adjustment of the clamping device, in practice it appears to be again a somewhat complicated, due to that its requires costly machining operations to provide the screw threads along the tie bars, which must consequently undergo expensive surface treatments in order to withstand the high contact forces, which would otherwise cause deformation and considerable wear.
Moreover, the time required for adjusting the stop members and for clamping the locking jaws, is still comparatively long with respect to cycle times in the range of seconds, for fast-cycle machines.
Lastly, U.S. Pat. No. 6,200,123 describes an injection molding press having two stationary platens, rigidly connected by four side tie bars, and an intermediate movable platen which is made to reciprocate in respect to the stationary platens of the press.
The movable platen is urged against one of the stationary platens by a central hydraulic device comprising a force-transmitting tie bar which extends through the hollow piston of a hydraulic control cylinder designed exclusively to generate the clamping force. A locking bush is mechanically connected to rotate in respect to the piston of the hydraulic cylinder, and is provided with axial rows of teeth to engage with corresponding axial rows of teeth on the central tie bar; the mold thickness adjustment is performed by screwing in and out the clamping bush to change the clamping position exclusively in relation to the pitch between the teeth of the tie bar.
This solution also substantially presents the same drawbacks as the previous devices.